The present invention relates to a solar energy collection system and particularly to controls therefor.
In a conventional solar energy collection system a solar receiver intercepts, absorbs and converts solar radiation or insolation into sensible heat or useful thermal energy. A thermal load operatively coupled to and in heat exchange relation with the receiver utilizes the thermal energy to do work. Normally, heat exchange is accomplished by a forced circulation system including a pump for circulating a working fluid between the receiver and the load.
Typically, in a solar energy collection system, there is provided a control system including means for sensing certain critical parameters and means responsive to the sensed parameters to operate various components in accordance with a control strategy. One such critical parameter is the temperature of the receiver or more specifically an absorber surface located therein. It should be obvious that the absorber becomes heated to a useful temperature when there is sufficient sunlight. Thus, the temperature of the absorber can be sensed to determine whether or not there is sufficient input energy to start up the pump for circulating the working fluid. As sunrise, sunset and cloud cover vary throughout the year, absorber temperature sensing means may be used as a direct solar control.
In conventional systems, direct temperature sensing means have been used for the purpose of determining the temperature of the absorber. Thermocouples, resistance thermometers, bimetallic strips, and fluid expansion detectors are exemplary of devices which have been used for this purpose in the past. However, they suffer from the difficulty that they must be attached to the absorber or inserted into the receiver. The ability of such devices to actually detect the true temperature of the absorber is in question. Instruments such as pyranometers and pyroheliometers and the like that directly measure solar input have been used, but they are expensive. In the present invention linear expansion of the absorber is used to produce an accurate temperature indication.
It is well known that metal expands when heated. Copper, for example, has a linear expansion of 178.times.10.sup.-7 "/"/.degree.C. Thus, a sufficiently long absorber plate will produce a visible lengthening when heated. In the present invention the expansion of the absorber is detected and used to accurately infer temperature. The present invention is especially adapted for use with relatively long evacuated tubular collectors, hereinafter described, wherein the lengthening of the absorber may be observed without inordinately sensitive detection equipment.
A typical evacuated tubular collector utilizes a flat rectangular absorber plate and a "U"-tube welded to one side thereof. The absorber and U-tube arrangement are located within an evacuated glass envelope transparent to solar radiation. The U-tube is a pipe bent on itself passing into and out of the envelope at one end thereof via glass to metal seals. The absorber is supported within the envelope by clips and the like. Solar radiation is intercepted and absorbed by the absorber, converted to thermal energy, and transferred by conduction to the U-tube and a working fluid passing therethrough. Although the evacuated tubular collector is preferred because it is believed to be the most efficient, it is possible to utilize the present invention with other types of collectors.
In the evacuated tubular collector, described above, it has been determined that for reasons of thermal and industrial efficiency a long, large diameter envelope would be desirable. The absorber, U-tube, glass-to-metal seals and the like being the same as above described and the absorber and U-tube being supported edgewise by clips or supports bearing against the inside wall of the envelope.